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CPSC 3730 Cryptography. Chapter 11, 12 Message Authentication and Hash Functions. Message Authentication. message authentication is concerned with: protecting the integrity of a message validating identity of originator non-repudiation of origin (dispute resolution)

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## CPSC 3730 Cryptography

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**CPSC 3730 Cryptography**Chapter 11, 12 Message Authentication and Hash Functions Cryptography**Message Authentication**• message authentication is concerned with: • protecting the integrity of a message • validating identity of originator • non-repudiation of origin (dispute resolution) • will consider the security requirements • then three alternative functions used: • message encryption • message authentication code (MAC) • hash function Cryptography**Security Requirements**• disclosure • traffic analysis • masquerade • content modification • sequence modification • timing modification • source repudiation • destination repudiation Cryptography**Message Encryption**• message encryption by itself also provides a measure of authentication • if symmetric encryption is used then: • receiver know sender must have created it • since only sender and receiver now key used • know content cannot of been altered • if message has suitable structure, redundancy or a checksum to detect any changes Cryptography**Message Encryption**• if public-key encryption is used: • encryption provides no confidence of sender • since anyone potentially knows public-key • however if • sender signs message using their private-key • then encrypts with recipients public key • have both secrecy and authentication • again need to recognize corrupted messages • but at cost of two public-key uses on message Cryptography**Fig. 11.1 Basic Uses of Message Encryption**Cryptography**Message Authentication Code (MAC)**• generated by an algorithm that creates a small fixed-sized block • depending on both message and some key • like encryption though need not be reversible • appended to message as a signature • receiver performs same computation on message and checks it matches the MAC • provides assurance that message is unaltered and comes from sender Cryptography**Message Authentication Code**Cryptography**Fig. 11.4 Basic Uses of Message Authentication Code (MAC)**Cryptography**Fig. 11.5 Basic Uses of Hash Functions**Cryptography**Message Authentication Codes**• as shown the MAC provides authentication • can also use encryption for secrecy • generally use separate keys for each • can compute MAC either before or after encryption • is generally regarded as better done before • why use a MAC? • sometimes only authentication is needed • sometimes need authentication to persist longer than the encryption (eg. archival use) • note that a MAC is not a digital signature Cryptography**MAC Properties**• a MAC is a cryptographic checksum MAC = CK(M) • condenses a variable-length message M • using a secret key K • to a fixed-sized authenticator • is a many-to-one function • potentially many messages have same MAC • but finding these needs to be very difficult Cryptography**Requirements for MACs**• taking into account the types of attacks • need the MAC to satisfy the following: • knowing a message and MAC, is infeasible to find another message with same MAC • MACs should be uniformly distributed • MAC should depend equally on all bits of the message Cryptography**Using Symmetric Ciphers for MACs**• can use any block cipher chaining mode and use final block as a MAC • Data Authentication Algorithm (DAA) is a widely used MAC based on DES-CBC • using IV=0 and zero-pad of final block • encrypt message using DES in CBC mode • and send just the final block as the MAC • or the leftmost M bits (16≤M≤64) of final block • but final MAC is now too small for security Cryptography**Fig. 11.6 Data Authentication Algorithm**Cryptography**Hash Functions**• condenses arbitrary message to fixed size h = H(M) • usually assume that the hash function is public and not keyed • cf. MAC which is keyed • hash used to detect changes to message • can use in various ways with message • most often to create a digital signature Cryptography**Hash Functions & Digital Signatures**Cryptography**Requirements for Hash Functions**• can be applied to any sized message M • produces fixed-length output h • is easy to compute h=H(M) for any message M • given h is infeasible to find x s.t. H(x)=h • one-way property • given x is infeasible to find y s.t. H(y)=H(x) • weak collision resistance • is infeasible to find any x,y s.t. H(y)=H(x) • strong collision resistance Cryptography**Simple Hash Functions**• are several proposals for simple functions • based on XOR of message blocks • not secure since can manipulate any message and either not change hash or change hash also • need a stronger cryptographic function (next chapter) Cryptography**Hash and MAC Algorithms**• Hash Functions • condense arbitrary size message to fixed size • by processing message in blocks • through some compression function • either custom or block cipher based • Message Authentication Code (MAC) • fixed sized authenticator for some message • to provide authentication for message • by using block cipher mode or hash function Cryptography**Hash Algorithm Structure**Cryptography**Secure Hash Algorithm**• SHA originally designed by NIST & NSA in 1993 • was revised in 1995 as SHA-1 • US standard for use with DSA signature scheme • standard is FIPS 180-1 1995, also Internet RFC3174 • nb. the algorithm is SHA, the standard is SHS • based on design of MD4 with key differences • produces 160-bit hash values • recent 2005 results on security of SHA-1 have raised concerns on its use in future applications Cryptography**Revised Secure Hash Standard**• NIST issued revision FIPS 180-2 in 2002 • adds 3 additional versions of SHA • SHA-256, SHA-384, SHA-512 • designed for compatibility with increased security provided by the AES cipher • structure & detail is similar to SHA-1 • hence analysis should be similar • but security levels are rather higher Cryptography**Table 12.1 Comparison of SHA Parameters**Cryptography**Fig 12.1 Message Digest Generation Using SHA-512**Cryptography**Fig 12.2 SHA-512 Processing of a Single 1024-Bit Block**Cryptography**SHA-512 Compression Function**• heart of the algorithm • processing message in 1024-bit blocks • consists of 80 rounds • updating a 512-bit buffer • using a 64-bit value Wt derived from the current message block • and a round constant based on cube root of first 80 prime numbers Cryptography**Fig. 12.3 SHA-512 Round Function**Cryptography**Fig. 12.4 Creation of 80-word Input Sequence for SHA-512**Processing of Single Block Cryptography

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